A well-known example of a device as defined in the preceding paragraph is a device for disinfecting water, in which the element which is arranged inside the housing is a source such as a lamp for emitting ultraviolet light, in particular of the type commonly referred to as UV-C. Exposing infected water to the ultraviolet light has a purifying effect on the water on the basis of the fact that the UV-C light is capable of killing germs. For domestic applications, the ultraviolet source is usually enclosed in a non-opaque holding structure, enabling the desired exposure of the water to the source.
For sufficient germicidal action, the ultraviolet disinfecting source has to produce a certain ultraviolet dose, expressed in J/m2. The dose is given by the irradiance (W/m2) multiplied by a residence time (s) of the bacteria in the device. The residence time as mentioned is determined by the flow paths of the water, and the irradiance level by the type of ultraviolet source which is applied.
When designing a water disinfecting device as mentioned, it has to be kept in mind that the irradiance of the ultraviolet source decays linearly, or, depending on the extent to which absorption takes place, linearly and exponentially with the radial distance to the source. In view of this fact, in order to achieve a desired effectiveness of the device, it is advantageous if measures are taken to ensure that radial mixing of the water elements takes place. By providing for radial mixing it is intended that all water elements pass the ultraviolet source on a very short distance at some point. Irradiance levels close to the source are so large that a short exposure of the bacteria in the vicinity of the source is sufficient to eliminate them.
Hence, a general problem that is encountered in the field of water disinfecting devices is that radial mixing is required to achieve a required dose in a limited exposure time or compact design.
According to a known possibility of enhancing radial mixing, a swirling flow is created by installing mixers upstream of the ultraviolet source, or by designing the inlet such that a spiraling motion occurs. This possibility is known from EP 803472, US 2008/0095661 and EP 616975. However, as the rotation is only imparted on the water at the inlet side, it will decay downstream. As a result, the degree of mixing is not constant over the length of the ultraviolet source, and the dose output is not optimal.
According to another known possibility of enhancing radial mixing, an inner wall of the housing is modified such as to be capable of creating a mixing effect in water that passes the wall. In this respect, U.S. Pat. No. 5,503,800 teaches the creation of grooves in an axial direction. However, there may be “dead zones” between the grooves, i.e. zones where the desired mixing effect does not occur, and where the water flows in an axial direction, more or less parallel to the ultraviolet source. As such flow paths are at a relatively large distance from the source, the required dose is not met. EP 202820 discloses another way of enhancing radial displacement of the water elements, namely applying ridges to restrict a flow in the axial direction. However, in that case, mixing is not optimal, as water is forced to flow towards the ultraviolet source at the expense of a higher velocity and thus shorter residence times.
Another problem that is encountered in the field of water disinfecting devices is that so-called short-cuts may be present. Short-cuts are flow paths leading directly from the inlet to the outlet. Bacteria following these short-cuts have very short residence times. Especially when the short-cuts prevail at an outer radius of the section where the ultraviolet source is arranged, at which location irradiance levels are lowest, very low dose levels result. Short-cuts are generally caused by certain inflow conditions and/or inappropriate mixing.
A known way of eliminating short-cuts involves placing a helically-shaped component in the section where the ultraviolet source is arranged, wherein the positioning of the component with respect to the ultraviolet source is such that the source is extending in the centre of the component. US 2003/0049809 discloses an example of such a component. When the component is applied, the water is forced to follow the helical shape, so that short-cuts are avoided. However, when the water follows a path around the ultraviolet source which is perfectly helical, the radial distance of the bacteria to the source does not vary while traveling through the section where the source is arranged. Therefore, there is actually no radial mixing, and no increase in dose output can be expected.